Various catalysts have been developed for eliminating substances harmful to humans and alleviating burdens on environments. In particular, catalysts which efficiently eliminate or detoxify harmful substances such as hydrocarbons (hereinafter referred to as “HC”), carbon monoxide (hereinafter referred to as “CO”), and nitrogen oxides (hereinafter referred to as “NOx”), for example, contained in exhaust gases emitted from internal combustion engines of cars and the like, thereby cleaning the exhaust gases have been developed. Those proposed as such exhaust gas cleaning catalysts include three-way catalysts which simultaneously clean HC, CO, and NOx at a stoichiometric air fuel ratio, thereby detoxifying them, and NOx occlusion and reduction catalysts which occlude NOx onto the catalysts in an oxidizing atmosphere and then reduce the NOx to N2 in a rich atmosphere. Since temperature conditions under which these catalysts are used include a high temperature on the order of 600 to 1100° C., it has been demanded to develop a catalyst whose catalytic activity does not decrease remarkably even when exposed to such a high temperature.
Proposed in response to such a demand are exhaust gas cleaning catalysts which, even when used for a long time in a high temperature environment, suppress the sintering of catalyst carriers and improve the chemical stability of active metal species supported by the catalyst carriers. For example, Japanese Patent Application Laid-Open No. HEI 9-141098 discloses an integral catalyst having the following configuration as an exhaust gas cleaning catalyst having a better durability at a high temperature than that of conventional catalysts and exhibiting excellent low temperature activity and cleaning performance even after being used for a long time at a high temperature.
Namely, the integral catalyst includes a carrier such as cordierite monolith and a catalyst component carrying layer, coated and fired on a carrier surface, supporting rhodium, whereas the catalyst component carrying layer contains a zirconium oxide represented by the general formula of [X]aZrbOc (where X is at least one species of element selected from the group consisting of magnesium, calcium, strontium, neodymium, yttrium, and lanthanum, whereas a, b, and c are respective atomic ratios of their corresponding elements; when b=1.0, a=0.01 to 0.6, and c is the number of oxygen atoms necessary for satisfying the atomic values of the components mentioned above).
In this integral catalyst, the added X element is completely dissolved as a solid in a crystal structure of the zirconium oxide, whereby the catalyst component carrying layer supporting rhodium particles is a zirconium oxide having a specific composition. Namely, no oxides made of the added elements alone exist in the catalyst component carrying layer supporting rhodium particles, so that inactive compounds are fully prevented from being formed by rhodium and oxides made of the added elements alone, whereas the structural stability improves at a high temperature, whereby a zirconium oxide having a large specific surface area can be obtained.